I hired a contractor to cut down trees and they drove into the backyard of my neighbor in doing the work. It was without my knowledge and certainly without my permission (since I cannot give anyone permission to go onto someone else's property),

The neighbor is now trying to say that their septic field may be damaged and have asked 2 companies to take a look.

Any thoughts.

Whatever the cost the contractor is liable. The neighbor should contact him and try to settle but you are in no way part of this or liable. I doubt if a 2 ton truck would damage a field but it might. What makes the neighbor think there is a problem, does he have a back up?

The neighbor did not indicate that he had any problem with respect to this. All he said by phone is that there were tire tracks on his yard and that there were some punctures from the falling of the tree.

I would also mention that we have had 5 inches of rain in the last few days. The water looks like a river coming from the highest point of his yard.

Thank you for your information.

You can always tell him you will pick up his expenses just to keep a good neighbor on your side. However, legally you are not responsible for any damage. The truck was driven by a contractor who carries his own insurance for the conduct of his employees. If your tree fell and puncture his drain field its his insurance that may cover it, not yours. Once a tree crosses a property line, by act of God, the tree is no longer your responsibility or property. Good luck.

The contractor is SUPPOSED to have insurance. GOD took the tree down?? I am not a lawyer but I don't think this is as clear cut as ballenger does.
A lot of maybes here. We don't know the laws in your jurisdiction. We don't know if the insur. Co. will pay without substantial proof that the tracks are theirs, that the system wasn't already damaged, etc. We don't know if it was even possible to do the work without infringing on the neighbor. At minimum, the tree contractor, his insurance rep, and you (with camera handy)should be represented when the system is inspected by folks who are probably in the business of repairing damage to septic systems.
Could a 4000 pound truck damage septic lines-absolutely-- whether the 5 inches of rain came before or after the episode.
This was a small pickup truck?
Here are some facts about vehicle weights (from cars.lovetoknow.com):
Is There An Average Weight For Cars?
"If you're wondering if there is an average car weight, the answer is yes. Compact cars range in weight from 3,000 to 4,500 pounds. Mid-size cars and sedans have a weight range of 4,500 pounds to 5,500 pounds depending on model. Light trucks, minivans and sport utility vehicles or SUVs can weigh between 5,000 pounds to 7,000 pounds. The larger pick-up trucks range from 7,500 to 12,000 pounds. Keep in mind that these are not gross vehicle weight ratings but gross vehicle weights."

If this becomes a lawsuit, I believe your name will be on it along with the contractor. You may be able to get your name removed but it may cost as much or more than the cost of repairs to the septic system.

I apologize for the skepticism. I have managed contractors on highway and bridge work for 40 years and I have done a lot of research on the risks of hiring contractors for work on my property. You don't even say whether the contractor is licensed or not. Do you have a copy of his insurance certificate? Is the premium paid up?

I would want to know the answers to all of these questions--most of them before the work even started.